Refiner bleaching of high yield pulps



Sept. 16, 1969 w. 5. WEST REFINER BLEACHING OF HIGH YIELD PULPS Filed June 14. 196 6 Wm mm United States Patent 3,467,574 REFINER BLEACHING OF HIGH YIELD PULPS William B. West, Eureka, Calif., assignor to Crown Zellerbach Corporation, San Francisco, Caliii, a corporation of Nevada Filed June 14, 1966, Ser. No. 557,498 Int. Cl. D21b ]/16 US. Cl. 16224 6 Claims ABSTRACT OF THE DISCLOSURE A process of bleaching lignocellulose material with a reducing bleaching agent which utilizes multistage refiners. The material is introduced into a first refiner where the temperature of the material is increased so that outgassing of oxygen-containing vapors deleterious to reduction bleaching takes place through a vent in a vented passageway for the material leading from the first refiner to a final refiner. The reducing bleaching agent is added to the material just prior to introduction of the material to the final refiner so that high temperature and mixing in the final refiner causes rapid bleaching.

This invention relates to the bleaching of high yield pulp in refiners employing reducing bleaching agents. High yield pulps are pulps obtained by processes where the pulp yield is above about 70%, and include groundwood and semi-chemical pulps.

It is known to employ as bleaching or brightening agents for papermaking pulps certain reducing agents such as sulfurous acid, its salts (such as sodium and calcium sulfite and bisulfite), and the reaction products of sulfu'rous acid with zinc and sodium to form hydrosulfites. Such reducing bleaching agents, as, for example, the hydrosulfites, are employed because they are less expensive than other brightening agents and are useful where only a moderate brightness gain is required, such as in book and newsprint papers. Such brightness gains are about 4-6 GE units with treatments of 0.5% hydrosulfite and up to 8-12 GE units with l2% treatment. The term GE units as employed herein is defined as the percent of incident light of 457 millimicron wave length reflected by the surface under test with respect to that of magnesium oxide, as determined according to TAPPI method T218 and T452.

Of the reducing bleaching agents commonly employed in pulp bleaching, the hydrosulfites can be considered to be the most important. Practically all the newsprint manufactured on the West Coast of North America is made from groundwood bleached with zinc hydrosulfite.

There are various methods commonly employed in the art to effect the bleaching of high yield pulp with re ducing bleaching agents such as the hydrosulfites. It is known, for example, that zinc hydrosulfite may be added to the hot pulp flowing out of the grinder pit where grinder stones are employed in making the groundwood. The disadvantages of this system far outweigh the advantages, however, and include poor mixing, excessive aeration, and variable retention time. Poor mixing is particularly serious since unmixed hydrosulfite rapidly reacts with air present to give thiosulfate (S 0 bisulfite (H80 sulfate (S0 and probably some sulfite (S0 the resulting effect being an appreciable loss in bleaching power and an increase in corrosiveness in the pulping system. Also, there is some brightness reversion due to aeration during subsequent screening and deckering.

The most common current method of bleaching with reducing bleaching agents such as hydrosulfites is by employing the retention tower system. In such systems, freshly prepared hydrosulfite solution varying in concentration from 110% is applied at the suction side of the stock pump at the retention tower inlet. Agitation is provided by the stock pump and by agitators located within the tower for mixing, or a special mixer may be inserted between the pump and the retention tower for effecting the mixing. While such a bleaching system is satisfactory, it requires a large capital investment in tanks, agitators, pumps and so forth, and the time required for effecting the bleaching is lengthy.

One of the primary difiiculties with employing reducing agents such as hydrosulfites are their extreme reactivity with oxygen and their decomposition by auto-oxidationreduction. Equations illustrating these two reactions which compete with the pulp for the bleaching chemical are as follows:

Another competing factor for the bleaching chemical are impurities present in the water employed in the refining process, conventional hydrosulfite bleaching being carried out at a consistency of 3-5 Recently, a process has been described in the Back et al. Patent 2,963,395, wherein the hydrosulfite bleaching is effected at high pulp consistencies, i.e., greater than about 15%, whereby this latter source of contamination is reduced due to the decreased amounts of water present.

The auto-oxidation-reduction reaction of Equation 2 above is catalyzed by acidity, and takes place rapidly below pH 5 and practically instantaneously below pH 4.0. Oxidation of the hydrosulfite by either the pulp or air raises the hydrogen ion concentration which, in turn, accelerates the auto-oxidation-reduction reaction. It is seen therefore that the longer the time required to effect the bleaching, the greater the loss in hydrosulfite available for bleaching.

It is an object of the present invention to provide a process for bleaching high yield pulp employing reducing bleaching agents wherein the bleaching agent is contacted and reacted with the pulp at a very rapid rate.

It is a further object of this invention to provide a process for bleaching high yield pulp with reducing bleaching agents wherein the bleaching is substantially effected at elevated temperatures above about 212 F., whereby the bleaching reaction takes place at a very rapid rate.

It is still a further object of this invention to provide a process for bleaching high yield pulp employing reducing bleaching agents wherein the pulp is bleached at a high consistency whereby high pulp temperatures may be effected and contamination by metallic ions present in water is substantially reduced.

It is another object of this invention to provide a process for bleaching high yield pulp employing reducing bleaching agents wherein the presence of oxygen is substantially eliminated from the pulp.

It is a further object of this invention to provide a process for bleaching high yield pulp employing a reducing bleaching agent whereby the need for separate bleaching facilities is substantially eliminated.

These and other objects and advantages of this invention will be apparent from the following description and by reference to the accompanying drawing wherein:

FIGURE 1 is a schematic view of a bleaching system employing the process of this invention; and

FIGURE 2 is a detail of a bleaching refiner which is suitable for practicing the present process.

Briefly, the process of the present invention comprises at least partially defibering a lignocellulosic material at a high consistency, elevating the temperature thereof to a value that is high enough to effect outgassing of water vapor and air from within the pulp mass, yet not so high as to deleteriously affect the lignocellulosic material; feeding the resultant pulp along an enclosed passageway to a refiner; introducing a reducing bleaching agent into the pulp immediately prior to its introduction into the refiner; passing the mixture of pulp and bleaching agent into the refiner which is operated under conditions to impart to the mixture a temperature that is substantially above about 212 F., yet below that temperature at which the mixture or its components are deleteriously affected for the period of exposure thereto, whereby the bleaching reaction is substantially completed during the traverse of the pulp through the refiner; discharging the pulp from the refiner without undue exposure thereof to air into a holding zone; and, substantially simultaneously with said discharging, diluting the pulp to a lower consistency. More specifically, the invention comprises employing a multi-stage refiner groundwood system wherein in the first stage the wood chips are passed through a first refiner where they are at least partially defibered at a consistency between about to and simultaneously heated from a temperature of about F. up to a temperature between about to 212 F.; conveying the pulp to a second refiner stage where the pulp is further refined; maintaining the pulp substantially at said elevated temperature and conveying it along an enclosed passageway to a third refiner stage; introducing the partially defibered pulp into the third refiner while introducing a reducing bleaching agent into the pulp immediately prior to its introduction into the refiner; reducing the consistency of the pulp coming from the third refiner to about 4% to 5%; and retaining the pulp in a storage zone for short period of time in order to complete the bleaching effect.

It is important that the pulp is maintained at an elevated temperature after the first refiner stage in order that ougassing of water vapor and air are effected from the pulp mass. By the term outgassing, it is intended to mean the condition wherein water vapor and/or other entrained gases within the pulp mass leave the pulp mass, thereby becoming de-entrained. The sumps and conveyors which hold and carry the pulp are enclosed in order that exposure to air is minimized. However, the sumps and conveyors are suitably vented to the atmosphere so that the water vapor and air that are outgassed do not create a pressure above the pulp mass that is substantially greater than atmosphere. By virtue of such outgassing of water vapor, the entrainment of air into the pulp is substantially prevented, and any air already entrained is substantially eliminated by a self-purging action. This is critical to the effective operation of the system as the presence or introduction of substantial amounts of oxygen into the pulp would rapidly destroy the effectiveness of the reducing bleaching agent. It has been found that for effective operation, wherein the system is substantially at atmospheric pressure, the temperature of pulp should be maintained as high as practical up to about 212 F., and not less than about 150 F., preferably not less than about F. While temperatures above about 212 F. may be employed, the danger of deleteriously affecting the pulp is thereby increased. The actual temperature chosen depends upon several factors, such as the pulp consistency and the length of the passageway from the first refiner holding zone to the second refiner. For a particular temperature, the length of this passageway must be great enough to permit the expression of substantial amounts of any entrained air, yet should not be so long as to risk fresh exposure to air of to permit a decrease in temperature of the pulp mass to the point where outgassing no longer exists. Supplemental heating of the conveyor may be employed, such as external heating or introduction of steam into the conveyor.

While the effective temperatures just mentioned are desirably effected by virtue of the work done upon the lignocellulose in the first stage refiner, it may be partially or wholly accomplished by separate conventional heating procedures, as long as they are of such a nature as to not permit undue exposure of the lignocellulose mass to air.

It should be further noted that the system may be maintained under a pressure less than atmospheric, in which case the temperatures necessary to effect outgassing from within the pulp mass are less than those set forth above. Also, inert gases may be employed in-the system to minimize oxidation. Such systems would be more costly than the system just described, however, and they are not necessary for the effective operation of the present process.

While in the foregoing general description of a threestage refiner groundwood system it is stated that the reducing bleaching agent is introduced into the third stage refiner, it is also possible to introduce the bleaching agent into the second refiner of the refining system. Regardless of the number of stages employed in a particular refiner groundwood system, however, it has been found to be more desirable to add the bleach to the last stage to obtain maximum benefits. By employing the bleaching process of this invention, it is possible to raise the brightness of the high yield pulp from about 8 to about 14 GE brightness units above the unbleached brightness value.

The bleaching is effected substantially instantaneously in the refiner. The high temperatures which are effected momentarily within the working space of the refiner permit the bleaching reaction to take place extremely rapidly, and yet these temperatures are maintained for such a short duration that detrimental effects to the pulp and to bleaching agent are obviated. The actual temperature that the pulp attains in the bleaching refiner is not determinable; however, based upon theoretical heat balance considerations which take into account the mass of the pulp involved, the pulp consistency, the amount of energy applied to the system and the pressure applied to the refiner plates, it appears most likely that the temperature is well above the boiling point of water at atmospheric pressure, i.e., above about 212 F., and may be as high as about 450 F.

Immediately upon discharge from between the plates of the refiner, the pressure upon the pulp mass is dropped to atmospheric, and some water is flashed off as steam which causes a cooling of the pulp. In addition, dilution water is added to the pulp substantially immediately after discharge from between the refiner plates which further aids in cooling the pulp down to a nondeleterious temperature range slightly below the boiling point of water.

After discharge from the refiner in which the bleaching has been effected, the bleached pulp is discharged into a sump from which air is substantially excluded. Simultaneously, the consistency of the lPlllP is reduced to about 4%- 5% by the further addition of dilution water. While pulp coming from the bleaching refiner has been tested for the presence of reducing bleaching agent and found to contain little in most instances, it has been found that some finite retention time after the refiner bleaching is effected appears to be desirable in order to fully develop maximum brightness. The reason for this is not fully understood. Since the refining process is generally run on a continuous basis, such retention time presents no storage problem as the pulp is continuously introduced and withdrawn from the refiner sump for an average retention time sufficient to allow completion of the bleaching action. An average retention time of about twenty minutes has been found to be sufficient.

The lignocellulosic materials employed in the practice of the present invention comprises any woody or nonwoody lignocellulose which may be partially or wholly defibered in a high yield refiner system. Exemplary of suitable woody materials are hemlock and Douglas fir. Exemplary of a suitable nonwoody material is bagasse.

Referring now to the drawing, wood chips or sadwust are fed from a source 10 via conduit 11 to a storage silo will 12. The chips or sawdust may be heated or unheated. From silo 12 the chips or sawdust are conveyed via enclosed conduit 13 by means of screw conveyors (not shown) to an enclosed screw conveyor 14. Screw conveyor 14 meters the chips or sawdust to outlet 15 which communicates with refiner 16 via duct 17. Excess chips or sawdust are returned to storage silo 12 via conduit 18. Water is introduced into the chips via inlet 19 in an amount sufiicient to give a consistency of between about 15% to 50%. The use of a chemical agent to aid in the separation of the wood fibers in the refiner may be employed, such as the addition of sodium hydroxide as is employed in the cold soda process. The passage of the lignocellulosic material through refiner 16 at least partially defibers it and raises the temperature of the resulting pulp to between about 150 F. to about 212 F. This pulp is dropped into enclosed sump 20 located beneath refiner 16 via duct 21. Sump 20 is vented to the atmosphere by suitable valve means, not shown. From sump 20 the high consistency pulp is conveyed via enclosed conduit 22 by means of screw conveyors (not shown) to enclosed screw conveyor 23, which meters the pulp to opening 24 through which it is fed, via duct 25, into refiner 26. Screw conveyor 23 is vented to the atmosphere by suitable valve means, not shown. Excess pulp is returned to sump 20 via conduit 27.

The pulp is further heated and defibered in refiner 26, from which it is discharged into enclosed, but vented, sump 28 via duct 29. From sump 29 the high consistency pulp is conveyed via enclosed conduit 30 by means of screw conveyors (not shown) to enclosed screw conveyor 31, which meters the pulp to opening 32 through which it is fed, via duct 33 into refiner 34. Immediately prior to the introduction of the pulp into refiner 34, a reducing bleaching agent such as zinc hydrosulfite is introduced to the pulp via inlet 35.

Refiner 34, as well as refiners 16 and 26, may be any suitable conventional refining apparatus which has two opposed surfaces which are spaced apart and moving relative. to one or the other, suchas conventional single disc, double disc or conical-type refiners. The relative tangential velocity between the opposed surfaces in refiner 34 should be relatively high, i.e., greater than 1000 f.p.m. and preferably greater than about 5000 f.p.m. in order to cause the pulp and bleaching agent to move rapidly and continuously through the refiner so as to minimize exposure of these materials to the relatively high temperatures imparted thereto in the refiner.

Upon discharge of the pulp from between the plates of refiner 34, it is substantially immediately diluted with water introduced via inlet 37. The pulp is further diluted to a consistency of about 4%5% in sump 36 by water introduced via inlet 38. This immediate diluation of the pulp has been found to be desirable in order to prevent brightness reversion which has been found to occur to a certain extent if the pulp is stored at high consistency. The dilution water should be hot, in the neighborhood of 140- 160 F., in order to maintain outgassing from within the pulp mass whereby air is not easily entrained. Also, sump 36 into which the pulp is dumped via duct 39 from refiner 34 is enclosed, but vented, and kept continuously full in order to prevent or minimize contact with air.

From low consistency sump 36, the bleached stock is continuously withdrawn and fed to conventional screening means 40, cleaner 41, decker 42, and finally into storage tank 43. From storage tank 43 the bleached pulp is withdrawn for use in making paper by conventional procedure's. It has been found to be highly desirable to maintain the temperature of the pulp above about 150 F. during the screening and cleaning operations to prevent any substantial brightness reversion.

FIGURE 2 shows in detail the construction of bleach ing refiner 34. Refiner 34 is a conventional doublerevolving disc refiner which has been modified for use in the present process. This refiner is substantially the same in principle as disclosed in US. Patents 2,214,707 and 2,568.783.

Refiner 34 comprises a pair of rotatable refining discs 51 and 52 carrying removably mounted,,roughened surface, refining plate sections 53. The working space 54 between the discs is usually tapered toward the periphery. The partially defibered pulp to be bleached is introduced into working space 54 in the usual manner from duct 33 through inlet 55 adjacent the axis of rotation of the discs, and between spokes 56 supporting disc 52. Discs 51 and 52 are rotatable in opposite directions about a fixed common axis by suitable power means. Disc 52 is rotatable in a fixed plane while disc 51 is yieldably mounted for slight movement along the axis of rotation of the disc. The minimum spacing between the disc may be adjusted by conventional means, not shown.

Reducing bleaching agent is introduced into the pulp stream being fed into refiner 34 via inlet 35. As is seen from FIGURE 2, the reducing bleaching agent is not contacted with the pulp until just prior to its introduction into the Working space of the refiner.

The bleached pulp leaves working space 54 at the periphery of the discs, and drops to. the bottom of the housing or refiner 34, through duct 39 and into sump 36 located therebelow. Hot dilution water is introduced into the housing of refiner 34 via conduits 37 (a) and 37 (b).

In order to improve the stability of the hydrosulfite to iron present in the pulp or in the process water, certain stabilizers may be incorporated into the pulp prior to incorporation of the reducing bleaching agent itself. It is also possible to add the stabilizers to the dilution water after bleaching, but this is less desirable than addition prior to bleaching. Examples of such suitable stabilizers for hydrosulfites are the water soluble polyphosphates, such as sodium tripolyphosphate, sodium tetraphosphate, tetra sodium pyrophosphate, chelating agents such as ethylene-diamine-tetra-acetic acid or the alkali metal, alkaline earth metal and ammonium salts thereof, alkali metal citrates and the like. In general, the stabilizer is employed in amounts of from about 0.01% to 2% and preferably from 0.05% to 0.5% by Weight based on the oven dry weight of the pulp. The amount of stabilizer may be somewhat less if it is a chelating agent rather than a polyphosphate-type stabilizer.

If the reducing bleaching agent employed is zinc hydrosulfite, it may be prepared by the reaction between zinc dust and sulfurous acid as is disclosed in copending application Serial No. 416,805. The hydrosulfite is used as an aqueous solution at a concentration of less than about 10% by weight and preferably about 6.0%, and in amounts of up to about 4% by weight of hydrosulfite based upon the oven dry pulp. The pH of the hydrosulfite may range from about 4.5 to about 12 depending upon the type of hydrosulfite employed and the acidity of the pulp to be bleached. However, it is usually desirable to have the pH in the neighborhood of 5.0.

The optimum pH range for brightening pulp with zinc hydrosulfite is between 4.5 and 6, and with sodium hydrosulfite between 6 and 8. The particular pH level employed, however, is the result of numerous factors such as pitch control, wood species, and pH and chemical composition of the process water employed. Generally, no buffer is added for pH adjustment during zinc hydrosulfite brightening except in the case of highly acid wood species. As previously mentioned, it is desirable to maintain a pH above about 4.5 as below this pH the auto-oxidationreduction reaction takes place very rapidly, and practically instantaneously if the pH is below about 4.0.

As previously mentioned, one of the important features of the present invention is the utilization of very high temperatures, which may be effected in a refiner such as the disc-type, to cause the bleaching reaction with the reducing bleaching agent to take place substantially instantaneously. In conventional bleaching processes employing reducing bleaching agents, the temperatures employed are from to F. The Back et al. Patent 2,963,395 shows that bleaching time decreases with increasing bleaching temperature, but only discloses that a maximum temperature of up to about 230 F. is employable. This was because at temperatures much above 230 F. the pulp and the bleaching agent were deleteriously affected due to the long exposure to these temperatures necessary in practicing the inventions described therein.

It has been found that employing a disc refiner in which the plates are urged toward each other under a large positive pressure by hydraulic pressure means, and by using pulp at a high consistency, theoretical temperatures of about 300-450 F. are obtainable, which said temperatures are affected only for the short period of time to which the pulp is passing through the refiner. Upon discharge from the refiner the pulp is substantially immediately diluted to a low consistency whereby the temperature is dropped back down to a non-deleterious range in the neighborhood of about 200 F. Since the reducing bleaching agent is introduced to the pulp immediately prior to introduction of the pulp into the bleaching refiner and the bleaching reaction takes place substantially instantaneously in the refiner, there is little chance for the bleaching agent to be exposed to air whereby it is oxidized and rendered ineffective.

It has further been found that this bleaching action may be effected at the high consistency necessary for obtaining the high pulp temperature in the bleaching refiner, and yet still obtain intimate contact between the bleaching agent and the fibers. The hydraulic pressure loading on the disc plates is desirably in the neighborhood of about 15 to 25 p.s.i., and the total power input between about to 60 horsepower days per air dry ton of pulp, in order to effect the high temperatures just referred to, and to effect intimate contact between the bleaching agent and the pulp fibers.

Ordinarily, the present process contemplates that the pulp entering the bleaching refiner will be at least partially defibered, but not wholly defibered as is the case in conventional bleaching processes employing reducing bleaching agents wherein the bleaching is effected in a separate operation from the defibering operation. However, the present bleaching process is also useful in those multistage refiner defibering procedures wherein substantially all of the defibering is effected in the first stage refiner, the bleaching being preferably effected in the last refiner stage of such procedures, as heretofore discussed. The important feature of the present invention in this regard is that the bleaching is effected as an integral operation in those refiners already employed in the primary preparation of the fibers for papermaking, whether by way of defibering, fibrillation and/or fractionation action upon the fiber bundles or individual fibers, and does not contemplate bleaching in a separate operation wherein a refiner is employed as a mixing device only and requires separate auxiliary equipment such as tanks, pumps and so forth.

The following examples illustrate the invention, but are not to be construed as limiting the scope thereof.

EXAMPLE 1 The system illustrated in FIGURE 1 was employed. The refiners were all double-revolving disc Bauer 480 refiners. Hemlock chips were fed by conveyor to the No. 1 disc refiner and introduced therein along with sufficient water to give a consistency of about 25%. The partially defibered pulp was discharged from the No. 1 refiner into the N0. 1 sump at a consistency of about 25% and a temperature of about 170 F. The No. 1 sump was suitably vented in order to permit the escape of steam therefrom. From the No. 1 sump the pulp was fed along an enclosed screw conveyor provided with suitable venting means to the No. 2 disc refiner, and, from there, to the No. 3 disc refiner, as illustrated. Zinc hydrosulfite liquor at a concentration of about 6.0% by weight was metered to the groundwood pulp in the eye of the No. 3 refiner. The zinc hydrosulfite was introduced to the pulp at varying levels of 0.6%, 1.0% and 1.6% by weight of the dry pulp. The amounts of zinc hydrosulfite introduced do not substantially change the consistency of the pulp in the refiner from the 25% level. The energy input to the refiner was about 35 horsepower days per ton. The bleached pulp was dumped from the No. 3 refiner into the No. 3 sump which was maintained full in order to prevent air mixing into the pulp with the agitator means employed therein. The average retention time of the pulp in the sump from which pulp was continuously being withdrawn and introduced was approximately 20 minutes. Samples of pulp were taken before the No. 2 refiner and after the 20-minute retention time in the No. 3 sump. The results are as follows:

TABLE I Test Test Test Test No. 1 No. 2 No. 3 No. 4

Percent zine hydrosulfite 0. 6 1. 0 1. 6 4. 0 Unbleached brightness, GE- 53. 9 53. 9 53. 9 52. 3 Bleached brightness, GE 62. 4 62. 8 65. 1 66. 5 Points brightness gain.-- 8. 5 8. 9 11. 2 14. 2 Residual hydrosu1fite.. No No No Yes EXAMPLE 2 The effect of retention time on the brightness of the bleached pulp stored in the No. 3 sump at low consistency was determined. The procedure of Example 1 was followed under the conditions of Test No. 3. The bleached pulp was allowed to remain in the sump for 7 hours and brightness readings taken. The results are as follows:

TABLE II Retention time, hrs. at 158 F.

and 4% consistency: Brightness, GE 65.1 64.6

From the foregoing, it is seen that retention of hot bleached refiner groundwood pulp at low consistency does not adversely affect the brightness of the stock.

EXAMPLE 3 The effect of retention time on the brightness of bleached pulp stored in the No. 3 sump at high consistency was determined. The procedure of Example 2 was followed except that the pulp discharged into the No. 3 sump was not diluted, but stored at a consistency of about 20%. The amount of zinc hydrosulfite employed was 0.75% for Test No. 1 and 1.25% for Test No. 2. The results of these two tests are as follows:

\IMUJNHO TABLE III Retention time, min. at F. and 20% consistency Test No. 1:

Brightness 59. 1 58. 8 59. 6 58. 3 58.0 Residual hydrosn1fite N0 N o No N o No Test No. 2:

Brightness 60. 4 61. 2 60.3 60. 6 58. 8 Residual hydrosulfite Yes No No No N 0 From the foregoing results it is seen that too long retention at high consistency has a generally adverse effect upon the brightness. One possible explanation is that at high consistency if there is any air available initially it will remain available to adversely afiect the terminal bleaching action, whereas if the pulp is diluted, any such air is displaced by water.

EXAMPLE 4 Tests were run to determine the efiect of zinc hydrosulfite addition to the chip (No. 1) refiner. The chips employed were 50% Douglas fir and 50% hemlock. Zinc hydrosulfite in varying amounts was measured to both the No. 1 and the No. 3 refiners. Samples were taken after No. 1 and No. 3 refiners and from the conveyor between the No. 1 and No. 2 refiners. Results are as follows:

From the foregoing, it may be seen that the addition of zinc hydrosulfite to the No. 1 refiner is ineffective to bleach the pulp. This is probably due to the low temperature and large amount of air initially present in the chips introduced to the No. 1 refiner.

EXAMPLE 5 Tests were made to determine the extent of brightness reversion during the screening and cleaning operations. The procedure described in Example 1 was followed. The wood chips employed were hemlock, and the zinc hydrosulfite was metered into the No. 3 refiner to give an effective addition of 0.57% by weight. The results are as follows:

TABLE V Percent zinc hydrosulfite 0.57 Brightness, GE 0 Unbleached 51.8 From No. 3 refiner 61.9 From No. 3 sump 63.1 From decker 63.6 From eggar 61.3

While the foregoing results indicate that there is a small loss in brightness during screening and cleaning, the extent of loss is not significant.

From the foregoing it is seen that a process has been provided for bleaching groundwood pulp with a reducing bleaching agent employing existing primary refiner equipment and not requiring a large capital investment for a separate bleaching plant.

I claim:

1. A process of bleaching a high yield pulp with a reducing bleaching agent comprising the steps of:

(a) feeding a lignocellulose material at a temperature less than 212 F. and at a consistency of greater than 15% into a first refiner;

(b) raising the temperature of the material inside the refiner while at least partially defibering the material in the refiner to expose fibers for facilitating subsequent bleaching and discharging the material from the refiner into a vented, enclosed passageway;

(c) said temperature increase in the refiner being sufiicient to raise the temperature of the material in the passageway to between about 150 F. and 212 F. and thereby causing outgassing of vapors surrounding the material in the passageway;

(d) venting the passageway between the first refiner and a subsequent refiner for permitting vapor flow caused by said outgassing through the vent so that the vapors are removed from the material while preventing additional outside air from entering into the vent;

(e) introducing a reducing bleaching agent into the material;

(f) feeding the material and bleaching agent into said subsequent refiner;

(g) operating said subsequent refiner under conditions such that the material is exposed to temperatures substantially above about 212 F. but below that temperature at which the lignocellulose material is deleteriously affected for the time of exposure thereto, whereby the bleaching reaction between the lignocellulose material and bleaching agent is substantially completed by the time the material is discharged from said subsequent refiner; and then,

(h) discharging the substantially bleached material from said subsequent refiner.

2. The process of claim 1 wherein the consistency of the material fed to the first refiner is between about 15 and 50%.

3. The process of claim 1 wherein the reducing bleaching agent is zinc hydrosulfite.

4. The process of claim 1 wherein the temperature of the pulp being fed to said subsequent refiner is between about F. and 212 F.

5. The process of claim 1 wherein the pulp is diluted to a consistency below about 5% after discharge from the refiner.

6. The process of bleaching a high yield pulp with a reducing bleaching agent comprising at least partially but not wholly defibering a lignocelulose material at a consistency of between about 15% and 50% in a first disc refiner which performs sufficient work upon the lignocellulose material passing therethrough to raise its temperature so that the temperature of the material is between about 150 F. and 212 F. within a first, enclosed, vented passageway leading from the first refiner; feeding the resulting pulp along the passageway to a second disc refiner while maintaining the temperature of pulp substantially within said 150 F. to 212 F. range whereby outgassing of entrained air through the vent in the passageway is effected; passing said pulp through said second refiner; feeding the pulp from said second disc refiner along a second enclosed passageway to a third disc refiner, while continuing to maintain the temperature of the pulp within said 150 F. to 212 F. range; feeding the heated pulp into the working space of said third refiner; introducing zinc hydrosulfite at a solution concentration of less than about 10% by weight of solution into the pulp just prior to the introduction of said pulp into said working space, the amount of zinc hydrosulfite added being up to about 4% by weight of the pulp on an oven dry basis; performing sufiicient work upon the pulp in said working space to raise the temperature of the pulp therein to a temperature substantially above about 212 F., but below about 450 F.; and then diluting the pulp to a consistency below about 5% substantially immediately after the pulp leaves the working space of said third refiner.

References Cited UNITED STATES PATENTS 2,071,307 2/1937 Hirschkind 16271 2,963,395 12/1960 Back et al. 16283 3,023,140 2/1962 Textor 16226 3,388,037 6/1968 Asplund et al 16226 X S. LEON BASHORE, Primary Examiner R. D. BAJEFSKY, Assistant Examiner US. Cl. X.R. 162-26, 71, 83 

